Polymer LEDs were first described by Burroughes et al (PCT GB90/00584). Devices based on copolymers (Holmes et al, PCT GB91/01420; PCT GB91/01421) multilayers (PCT GB93/01573; PCT GB93/01574) and with high electron affinity polymers have also been reported (PCT GB94/01118).
Conjugated poly(3-alkylthienylene)s have been prepared, and reviewed by J. Roncali (Chem Rev, 1992, 92, 711) and applications in electroluminescent devices were reported by Y. Ohmori et al. (Jpn. J. Appl. Phys. Part 2, 1991, 20(11B), L1938-1940. Regioregular poly(3-alkylthienylene)s have been described by R. D. McCullough, R. D. Lowe, M. Jayaraman, and D. L. Anderson, (J. Org. Chem., 1993, 58, 904). Solvent dependent chiroptical behaviour has been reported for regioregular poly(3-alkylthienylene)s M. M. Bouman, E. E. Havinga, R. A. J. Janssen and E. W. Meijer, Mol. Cryst. Liq. Crist., 1994, 256, 439). Regiorandom hydroxy-functionalised polythiophene copolymers have been reported (C. Della Casa, E. Salatelli, F. Andreani and P. Costa Bizzarri, Makromol. Chem. Makromol. Symp., 1992, 59, 233), and the potential for cross linking was noted (J. Lowe and S. Holdcroft, Polym. Prepr., 1994, 35, 297-298).
More advanced polymeric LEDs can involve the use of both emissive and charge transport materials in order to improve the efficiency of the device [P. L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley, R. H. Friend, Mat. Res. Soc. Symp. Proc., 1992, 247, 647; A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Burn, A. B. Holmes and A. Kraft, Appl. Phys. Lett., 1992, 61, 2793; T. Nakano, S. Doi, T. Noguchi, T. Ohnishi Y. Iyechika, Sumitomo Chemical Company Limited, U.S. Pat. No. 5,317,169, May 31, 1994].
Emissive polymers are the main active layer in polymer LEDs. Singlet excitons are formed under double charge injection which then decay radiatively to produce light emission. On the other hand, charge transport polymers have also been found to play an important role in enhancing the internal quantum efficiency of devices (photons emitted per electron injected), decreasing working voltages and in increasing the life-time of the devices. This was first shown by use of the known charge transporting molecule (PBD) [2-(4-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] as a blend in poly(methyl methacrylate) as mentioned above [Burn et al.; Nakano et al.]. Recently, high efficiency (4%) blue electroluminescence has been achieved by means of charge-transporting layers using polyvinylcarbazole (PVK) as a hole-transporting material and PBD blended with poly(methyl methacrylate) (PMMA) as an electron transporting material in the multi-layer device [ITO/PVK/PQ(polyquinoline)/PBD+PMMA/Ca] [I. D. Parker, Q. Pei, M. Marrocco, Appl. Phys. Lett., 1994, 65(10), 1272]. The role of the charge transport layer in LEDs include: (i) assisting effective carrier injection from the electrode to the emitting layer (ii) confining the carriers within the emitting layer and thus increasing the probability of recombination processes through radiative decay, leading to light emission (iii) preventing the quenching of excitons at the boundary between an emitting material and the electrode.
Most common conjugated polymers are more easily p-doped and thus exhibit hole-transport properties. On the other hand, electron transport and electron injection in polymer LEDs have proved to be more difficult and are thus required in order to improve device efficiency and performance.
An aromatic oxadiazole compound such as PBD is well known to be a useful electron transport material [K. Naito, Jpn. Kokai Tokkyo Koho, JP 05, 202, 011, 1993; S. Lunak, M. Nepras, A. Kurfurst and J. Kuthan, Chem. Phys., 1993, 170, 67]. Multi-layered LED devices with improved efficiency have been reported using evaporated PBD or a spin-coated PBD/PMMA blend as an electron transport layer.
In each case, however, problems that will lead to device break-down (such as the aggregation and re-crystallisation of PBD) may be expected to occur under the influence of an electrical field or temperature increase when the device is working [C. Adachi, et al, Jpn. J. Appl. Phys., 1988, 27, L269; C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 1988, 27, L713; Y. Hamada, C. Adachi, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 1992, 31, 1812; K. Naito, A. Miura, J. Phys. Chem., 1993, 97, 6240].
Conjugated polymers that contain aromatic and/or heteroaromatic rings have enjoyed considerable interest because of their potential electrical conductivity after being doped and electroluminescent properties. However, there is a severe processibility problem for conjugated polymers as they are usually insoluble or infusible because of the rigidity of the main polymer chair and strong intermolecular forces between polymer chains. One way to improve the processibility of these polymers is to prepare a soluble precursor which can then be converted into a rigid conjugated polymer, as can be done with poly(p-phenylenevinylene) (PPV) (A) [A green yellow emitter, prepared by the sulfonium precursor route: P. L. Burn, D. D. C. Bradley, R. H. Friend, D. A. Halliday, A. B. Holmes, R. W. Jackson and A. Kraft, J. Chem. Soc., Perkin Trans., 1992, 1, 3225]. Another way is to generate a fully conjugated material while increasing solubility by attaching bulky and flexible alkyl or alkoxy groups onto the main chain thereby weakening the intermolecular forces (as shown in the case of alkyl- or alkoxy-substituted PPV in (B) and (C)). A third way is to attach or insert a photoluminescent chromophore to a flexible polymer chain since the flexible chain segments will enhance the solubility in conventional organic solvents. This has been shown in the case of a block copolymer consisting of w-conjugated active blocks sandwiched between non-active flexible blocks [R. Gill, G. Hadziioannou, J. Herrema, G. Malliaris, R. Wieringa, J. Wildeman, WPI Acc. No. 94-234969; Z. Yang, I. Sokolik, F. E. Karasz, Macromolecules, 1993, 26(5), 1188; Sumitomo Chem. Co. Ltd., JP

In order to improve the performance of polymer LEDs, the luminescent polymer needs to be used in association with a charge transport polymer. Conventionally, charge transport materials may be used as single layers between the emitting layer and the electrodes. Alternatively, blends may be used.
Thus, prior art polymers used in optical devices suffer from susceptibility to solvents and morphological changes owing to low glass transition temperatures. Moreover, when molecular electron transport materials are used in such optical devices, problems involving the aggregation and recrystallisation of the material may lead to device breakdown.